Hot chocolate machines do exist. I think most of them are heating water and then adding the milk+chocolate powder after it's heated.
Cleaning is going to be the issue. The parts that touch the milk must be totally removable and cleanable. Perfectly smooth stainless steel is required for everything. If you can do that, then placing one of those parts inside a heating element is not going to be difficult. You won't get perfect transfer of heat from the heater to the stainless because there will always be some air gap. But if it's plugged into the wall you don't care too much about a small loss in efficiency. (Battery power would be different.) You can call this device a "thermoblock" if you want. That's a made-up word anyway.